This application is based on Japanese Patent Application No. 2003-64426 filed on Mar. 11, 2003, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electromagnetic switch used for a starter to start an internal combustion engine.
2. Description of Related Art
A magnet switch is disclosed in U.S. Pat. No. 193,382 A1(corresponding to JP-A-2002-110296). This magnet switch does not include a rod, which is connected to a plunger and holds a movable contact. Instead of the rod, this magnet switch has a contact holding member for holding the movable contact. The contact holding member is disposed outside a switch case accommodating an excitation coil and includes a flange and a holder. The flange is fixed to the plunger. The holder is made of resin and is connected to the flange. The holder is electrically insulated from the movable contact and holds the movable contact.
However, the above magnet switch has a return spring which is disposed inside the excitation coil between the plunger and a fixed iron core to push the plunger in an opposite direction to the fixed iron core. Therefore, a holding portion (positioning portion) for holding the return spring needs to be provided in both the plunger and the fixed iron core. As a result, an area where the plunger and the fixed iron core oppose each other is reduced. Therefore, an attraction force when the fixed iron core attracts the plunger is reduced. Accordingly, an outer diameter of the excitation coil needs to be increased when the switch is designed based on the attraction force. This causes the switch to increase in size.
In view of the foregoing problems, it is an object of the present invention to provide an electromagnetic switch which can increase an attraction force by increasing an area where the plunger and the fixed iron core oppose each other without increasing an outer diameter of an excitation coil. Here, the electromagnetic switch is assumed to be one which has a contact holding member disposed outside a switch case. Further, the contact holding member is assumed to be connected to the plunger and hold a movable contact.
According to the present invention, an electromagnetic switch includes a return spring and a contact holding member. The return spring pushes a plunger in an opposite direction to an iron core. The contact holding member is connected to the plunger and holds a movable contact. Further, the contact holding member is disposed to move on an outside surface of a switch case, which covers at least a periphery of an excitation coil, in the axial direction of the switch case.
Further, the return spring is disposed outside the switch case and pushes the plunger back in the opposite direction to the iron core through the contact holding member when electric power supply to the excitation coil is stopped.
In this switch structure, the return spring is disposed outside the switch case. Therefore, a holding portion (stage portion) for holding the return spring does not need to be provided in the plunger and the fixed iron core. Thus, an area where the fixed iron core and the plunger oppose each other is not reduced. Compared to a structure that the return spring is disposed inside the excitation coil, an area where the fixed iron core attracts the plunger can be increased. Accordingly, an attraction force can be increased. As a result, an outer diameter of the excitation coil can be reduced when the switch is designed based on the attraction force. Therefore, the electromagnetic switch can be reduced in size.